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In debian/ubuntu, libc.so is located at a different place,
/lib/x86_64-linux-gnu/libc-2.23.so, so it outputs like this when testing:

  PING ::1(::1) 56 data bytes
  64 bytes from ::1: icmp_seq=1 ttl=64 time=0.040 ms

  --- ::1 ping statistics ---
  1 packets transmitted, 1 received, 0% packet loss, time 0ms
  rtt min/avg/max/mdev = 0.040/0.040/0.040/0.000 ms
  0.000 probe_libc:inet_pton:(7f0e2db741c0))
  __GI___inet_pton (/lib/x86_64-linux-gnu/libc-2.23.so)
  getaddrinfo (/lib/x86_64-linux-gnu/libc-2.23.so)
  [0xffffa9d40f34ff4d] (/bin/ping)

Fix up the libc path to make sure this test works in more OSes.

Committer testing:

When this test fails one can use 'perf test -v', i.e. in verbose mode, where
it'll show the expected backtrace, so, after applying this test:

On Fedora 26:

  # perf test -v ping
  62: probe libc's inet_pton & backtrace it with ping       :
  --- start ---
  test child forked, pid 23322
  PING ::1(::1) ...